I have a template I generally adapt slightly to send out to companies. Some you win some you lose. I've had units from many different companies though and made some good contacts, such as Synology, Crucial Memory, Kingston Tech, GoPro, Lindy, etc.
In fact Kingston contacted me just last week as they wanted me to review a unit for them, and Lindy have contacted me just today with 2 sets of headphones, one wired and one wireless, that they want me to review.
Sometimes they don't ask for the unit back either so I get to keep it, whereas other times they collect the item from me once I've finished the review.
Week before last Kingston sent me their 512GB USB Flash drive to review, which is worth a lot of money, but I could only keep for 7 days! Other items I can keep for much longer.
You'll find that once you build a relationship with a company you don't need to keep chasing. They will contact you once they have something for you to review etc.